It is well known that peak radio frequency voltages in the range of 2 KV to 6 KV are commonly generated across the coils and capacitors in high power NMR probes and that this necessitates components with DC breakdown voltages in the range of 10 KV to 25 KV. Rf currents are typically in the range of 10 A to 100 A. Total circuit inductance is typically in the range of 0.1 .mu.H to 0.5 .mu.H and total circuit quality factor Q is typically between 80 and 400. Hence, it is essential that the series resistance and inductance of the capacitors and the leads be kept as small as possible for optimum performance.
It is necessary to tune the resonant circuit rather precisely (within 1/4Q) to the desired resonant frequency and to match the impedance to that of the transmission line, typically 50 ohms. This is commonly accomplished with high Q, high voltage variable capacitors as shown in FIG. 1, in which C3 is referred to as the tune variable and C4 is referred to as the match variable. The purpose of C1 and C2 is to serve as a capacitive voltage divider so as to permit the use of higher inductance coils, thereby reducing the effect of losses arising from lead inductance, without generating excessive voltages across the variable capacitors. Unfortunately, the tuning range of this circuit is severely limited by the introduction of C1 and C2, and multinuclear operation can only be achieved by changing their values. Of course, in low power circuits, C1 may be replaced with a short circuit, and one then has only to change C2 to effect a major change in the resonant frequency. However, as C2 becomes quite large, additional capacitance will be required in parallel with C4. Various low voltage capacitor switching means have been used commercially in this manner and for this purpose in low power probes.
Multinuclear capability can be obtained in high power tuned circuits only by the introduction into the circuit of a three-pole non-magnetic, high power, low inductance switch, able to withstand high current while being compact and having low loss, as disclosed herein.